Abstract

Echo integration is an established method for stock estimation. However, this method is not free of errors like
every other measuring method. Especially the variation
between day and night behaviour of fish may lead to
large measuring errors. A new method is represented detecting such systematic errors, exemplified by investigations during the international hydroacoustic survey on the spring spawning herring in the Norwegian Sea. For this method all measured sA-values are sorted by starting time of the measuring unit distance. In order to reduce random influences a moving average over five time
intervals is computed. When displaying these values in
a diagram makes it is very easy to detect systematic errors
based on the differences in day-night behaviour. For
both species, herring and blue whiting, stock estimations
are calculated based on the measured sA-values and the results of the analysed trawl catches. The influence
of the differnt day and night behaviour of herring on the results of its biomass estimation is rather low. For blue whiting the measured values were about three time higher during day time than during night time. The result of this investigation should initiate a change of the evaluation procedure for stock estimation based on hydroacoustic measurements.